Here is a great image from the Cody Museum showing a Great War-era Colt M1911 (bottom) compared to a post-war M1911A1.
Via the Cody Museum
The A1 designation came about in 1924 and changes to the standard include a thicker front sight, an arched and checkered mainspring housing (to improve aim/ergonomics), a longer hammer spur (to help curb reports of slide bite on firing), a shorter trigger with checkering and relief cuts in the guard to help those with smaller hands, and a longer grip safety spur.